Macrocyclic polyester oligomer (macrocyclic oligoester, MPO) has unique properties that makes it attractive as a matrix-forming resin for engineering thermoplastic composites. MPO lends valuable characteristics to polymerized products, for example, high strength, high gloss, and solvent resistance. Furthermore, because certain MPO's melt and polymerize at temperatures well below the melting point of the resulting polymer, polymerization and crystallization can occur virtually isothermally upon melting of the MPO in the presence of an appropriate catalyst. The time and expense required to thermally cycle a tool is favorably reduced, because demolding can take place immediately following polymerization, without first cooling the mold.
MPO generally exhibits low melt viscosity. A low melt viscosity generally provides improved processing flexibility. For example, an MPO with a low melt viscosity can easily impregnate a dense fibrous preform. However, for certain low-pressure processes, such as rotational molding and powder coating, the melt viscosity of MPO may be too low to allow processing with existing process equipment. For example, the water-like viscosity of melted MPO may cause it to leak during rotational molding. In powder coating applications, the low melt viscosity of certain MPO's makes it difficult to control coating area and coating thickness, for example.
Use of a viscosity-enhancing agent along with an MPO during processing is generally not satisfactory, for example, because the agent may not mix well with the MPO, because a separate processing step is generally required to introduce the agent, and because the agent may have a deleterious effect on the properties of the material following polymerization of the MPO.
In addition to having low melt viscosities, MPO's are highly friable. This provides certain handling and processing advantages. For example, MPO may be easily crumbled, pulverized, or reduced to powder prior to and/or during processing. The friability of MPO provides a distinct advantage in low-pressure processes such as rotational molding and powder coating, where the form of the MPO prior to melting and polymerization is important. Adding agents to an oligomer to increase its melt viscosity generally results in a less friable mixture, making it difficult to use in processes such as rotational molding and powder coating.
Furthermore, in some applications, polymerization of MPO results in a brittle, highly crystalline material that is not as tough or ductile as desired. Modifying raw materials in order to increase the ductility and/or toughness of the resulting polymer may adversely affect the processibility of the raw materials, for example, by reducing the rate of polymerization.
Therefore, there exists a need for a raw material that is stable and friable under ambient conditions, that polymerizes and crystallizes isothermally, that has a sufficiently high rate of polymerization, and that can be used in low-pressure processes such as rotational molding and powder coating, without modification of existing equipment, to produce a product of sufficient toughness and ductility.